Up-converting phosphor (UCP) nanoparticles based on rare-earth doped oxides have recently attracted substantial interest in both academia and industry for the purpose of labeling biological specimens. A UCP is a material which when excited in the infrared, for example with a low intensity CW diode laser, emits a number of narrow emission lines in the visible or near infrared. Because the emitted light is up-converted to shorter wavelengths than the excitation wavelength, background scattering is virtually non-existent. However, achieving mono-disperse particles with a combination of sub-10 nm size, low aggregation, size uniformity and high crystallinity required for high up-conversion efficiency and efficient surface functionalization has proven elusive to the present.
The main advantages provided by UCP nanoparticle imaging are the lack of auto-fluorescence by the specimen, the absence of bleaching, multicolor imaging and the ability to perform high resolution cathodoluminescent imaging with a Scanning Electron Microscope (SEM). Currently a great impediment to using UCP's in biological labeling applications is their size, a point in which quantum dots and organic dyes have excelled. Solution-stable, unaggregated, bright up-converting, oxide-based phosphors have previously been demonstrated for sub-micron sizes.
Because of the high degree of crystallinity required for up-conversion, attempts to synthesize smaller oxide-based up-converting particles were hindered by the need for post-synthesis annealing at a temperature above 500° C., which often leads to undesired sintering of particles. However, the annealing step is not required for the non-up-converting UV or near UV excited phosphors.
Thus, while particles around 10-17 nm based on upconverting NaYF4 have recently been synthesized by sol-gel chemistry approaches, the phosphors are not water-soluble, and the toxicity is as yet undetermined. There remains a need for a method for producing highly crystalline mono-disperse up-converting phosphorescent oxide particles with a particle size on the order of less than 10 nm that are capable of forming stable suspensions or dissolving in a solvent, especially a biocompatible solvent.